Add http health-check for the api app

Also rename the 'elb' argument to 'simple' to be more descriptive.

The healthcheck is going to issue requests to `/_status?simple=true`
just to check that gunicorn is up and running - we don't need to go all
the way to the database, as this would have adverse impact when api is
under load.

app/status/healthcheck.py

@@ -11,7 +11,7 @@ status = Blueprint('status', __name__)
 
 @status.route('/_status', methods=['GET', 'POST'])
 def show_status():
-    if request.args.get('elb', None):
+    if request.args.get('simple', None):
         return jsonify(status="ok"), 200
     else:
         return jsonify(